If the disposal is of an asset where the Special Rate First Year Allowance has been claimed at 50% or where the allowance was not claimed in full, you'll need to go to the Special Rate Pool (or General Pool) and dispose of the portion of the asset in that pool.
For example, a company boughty an asset for £100,000 in accounting period ending 31/12/21, where it claimed the Special Rate First Year Allowance at 50%. The company sold the asset for £50,000 in the following accounting period. You would need to enter £25,000 as a balancing charge in a user-defined pool, and £25,000 as a disposal in the Special Rate Pool.